Various systems, such as cellular communication systems, utilize high frequency modulated signals to transmit data between different locations. The use of the high frequency modulated signal allows the data to be transmitted via a wireless link and therefore avoids the costly expense of running cable between the locations.
Typically, in such systems, an intermediate frequency signal is generated by a local oscillator. This first intermediate frequency signal is then up-converted (i.e., frequency multiplied) so as to produce a carrier signal having a frequency suitable for transmission via a wireless link, for example, Ku or C band. The carrier signal is then modulated in accordance with the data to be transmitted. Alternatively, the intermediate frequency signal can be modulated prior to up-converting the signal to the carrier frequency.
The use of the intermediate frequency signal provides numerous advantages. For example, in many communication systems the location of the antenna for transmitting and receiving modulated carrier signals is distant from the components generating and processing the data. By utilizing intermediate frequency signals to transmit data between components contained in the same location, the need for expensive cable, which would be necessary to transmit high frequency carrier signals with acceptable loss and leakage levels, is eliminated. Accordingly, wireless communication systems typically up-convert the intermediate frequency signal to the carrier frequency as one of the last steps in generating the modulated carrier signal.
A problem remains in that prior art systems typically utilize conventional phase lock loops (xe2x80x9cPLLxe2x80x9d) to up-convert the intermediate frequency signal. The use of such PLLs, while raising the frequency as required, also results in a substantial increase in the phase noise and thus the phase sitter associated with the signal. The increase in phase noise degrades the spectral purity (i.e., increases the noise floor) of the signal, which is undesirable.
Another problem with prior art systems pertains to the frequency deviation characteristics of the modulator (i.e., the frequency error or frequency variance generated by the modulator) utilized to superimpose the data to be transmitted on the intermediate frequency signal prior to the up-conversion process. Frequency deviation relates to the amount the output of the modulator deviates from the desired frequency. As such, it is desirable to minimize the frequency deviation as much as possible.
In prior art systems which modulate the intermediate frequency signal and then up-convert the signal to the carrier frequency, the frequency deviation or error is also multiplied by the same factor as the frequency, which can be on the order of 100 or greater. Specifically, the frequency error increases in accordance with ratio between the carrier and the intermediate frequency. As a result, the frequency of the modulated carrier signal is susceptible to undesirable variations.
Accordingly, there exists a need for an apparatus for generating a modulated carrier signal suitable for transmission via a wireless link which negates the foregoing problems.
The present invention relates to an apparatus for producing a low phase noise, high precision, modulated signal. Specifically, the invention comprises a novel synthesizer/modulator design which allows for the modulation and frequency multiplication of an intermediate frequency signal without a corresponding increase in the frequency deviation or phase noise of the signal.
Accordingly, the present invention relates to an apparatus for generating a modulated signal comprising a modulator operative for receiving a first signal and input data signals, and modulating the first signal in accordance with the input data signals so as to produce a modulated reference signal; a first frequency divider coupled to the modulator output, operative to reduce the frequency of the modulated reference signal by a predetermined factor; a signal generator operative to produce a second signal; a first mixer having a first input coupled to an output of the first frequency divider and a second input coupled to the output of the signal generator. The first mixer operates to frequency translate the modulated reference signal by an amount equal to the frequency of the second signal.
In addition, the signal generator of the present invention comprises a direct digital synthesizer (xe2x80x9cDDSxe2x80x9d) coupled to a phase lock loop which operates to up-convert the output signal of the DDS to the microwave region. The feedback network of the second phase lock loop utilizes frequency translation to down-convert the signal in order to close the loop.
As described in detail below, the apparatus of the present invention provides important advantages. For example, by down-converting the modulated reference signal output by the modulator, the frequency deviation of the modulated reference signal is also reduced by the same factor. Accordingly, inexpensive modulators operating in the microwave region can be utilized. Without such down-conversion, upon completing the up-conversion process, the frequency deviation of the modulated reference signal would exceed the frequency deviation rating of the modulator.
Furthermore, the down-conversion of the modulated reference signal also provides the added benefit that any error in the I/Q balance of the input data signals to the modulator is also reduced by the same factor.
In addition, by utilizing frequency translation in the feedback loop of the phase lock loop contained in the signal generator, the present invention increases the frequency of the output signal of the DDS without increasing the phase noise or spurious frequency components of the signal.
The invention itself, together with further objects and attendant advantages, will best be understood by reference to the following detailed description, taken in conjunction with the accompanying drawings.